The subject matter of the present invention relates generally to athletic shoes and in particular to multilayered sole athletic shoes of reduced weight having mid-sole layers of low density synthetic plastic foam material including an intermediate sole layer and a heel lift sole layer between a harder, wear-resistant outer sole layer and a shoe upper.
In athletic shoes and especially in track shoes used for competition, the weight of the shoe is a very important factor. In recent years, the use of fabrics made of nylon and other synthetic materials for shoe uppers has reduced the weight of athletic shoes but multilayered sole athletic shoes remained relatively heavy. U.S. Pat. No. 3,793,750 to Bowerman shows one such athletic shoe made with a nylon sandwich type fabric upper and cushioned midsole layer covered by a harder outer sole layer. While this shoe has a lightweight nylon upper, it employs conventional foam rubber for its mid-sole layers and therefore is not substantially lighter than other athletic shoes.
Previously U.S. Pat. No. 3,724,105 to Weight, disclosed that some non-athletic shoes have been made with foamed ethylene-vinyl acetate copolymer insoles within the shoe upper for greater comfort. However, such synthetic foam material has not heretofore been used in thicker mid-sole layers, which represent the major portion of the shoe weight, because it previously lacked sufficient durability and/or resilience and because it substantially reduces the lateral stability of the shoe.
U.S. Pat. No. 3,589,036 to Headricks shows a canvas upper tennis shoe which uses a non-foam, synthetic rubber material to replace vulcanized rubber in the shoe sole. The use of such synthetic rubbers has not appreciably reduced the weight of athletic shoes.
Some shoe designs leave apertures in various mid-sole layers and thereby incidentally reduce overall shoe weight to a small extent. For example, U.S. Pat. No. 3,290,081 to Bente has proposed that spiked track shoes used for jumping include an empty aperture in the heel lift sole layer to reduce heel injury. Such shoes are not substantially lighter than standard athletic shoes and lack sufficient heal cushioning for running shoes.